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purely Perl-based mail filter framework for the Courier MTA

Courier::Filter implements the Courier MTA's courierfilter interface as a
framework for mail filter modules. Authors of filter modules can concentrate
on writing the actual filter logic without having to care about the usual
low-level details of the courierfilter interface. Logging to various
facilities is also supported.

Courier::Filter allows multiple filter modules to be installed in stacked and
hierarchically grouped configurations. Also, modules' polarity can be
reversed, so some modules can be used for explicitly accepting messages while
others are used in the traditional way for rejecting messages.